Ax comes down with a terrible sickness, and since unlike humans he can't throw up, he ends up with a fatal swelling of one of his glands. While they wait for Ax to get to a certain body temperature so they can do brain surgery on him and save his life, they also get word that Aftran, the Yeerk Cassie formed an alliance with, has been found to be the founder of a Yeerk peace movement. Delivering this news is Illim, a Yeerk who lives in one of their teachers, and he is also part of the peace movement. Cassie has to figure out a way to infiltrate the Yeerk pool and get Aftran out before Visser Three interrogates her and bares all their secrets. Their first group mission goes bad when Jake gets sick, and it turns out they have all been exposed to Ax's flu. One by one they all fall ill, except Cassie. She has to go on this mission alone, and does so by doing the unthinkable: morphing into a Yeerk. Using the form of Illim, Cassie rescues her friend Aftran and rushes back in time to do brain surgery on Ax, but because she has no way of knowing Andalite brain structures, she needs Aftran's help to actually physically find the gland and cut it out. They work together, and rescue Ax, but finally they have to figure out what to do to save Aftran for good, since in only three days she will die of Kandrona starvation. They come up with an elegant solution for her: turning her into a humpback whale with morphing technology, and they emerge from this battle knowing that a peace movement amidst the Yeerks is going to survive.

It's clear that the Yeerk Aftran has managed to start a peace movement somehow, but it's not clear how she was answering for not being in her host, who was supposed to be a young girl named Karen planted to watch the girl's dad and report on him to the Yeerks. It was never explained why Aftran managed to survive as long as she did in the Yeerk pool without being in her host.

Cassie again refers to her human form as her "human morph," which is inaccurate.

It's interesting that when Cassie has to do this mission all on her own, she assumes a lot of the roles of the others: saying "let's do it!" like Rachel, making dumb jokes to herself like Marco, and leading the way/making decisions like Jake.

It's not clear why Cassie of all people was the one who never got sick. If the flu was so contagious, why didn't she or anyone else around them get it? Because it was a bit too convenient, this felt more constructed than it should.

Cassie, despite her experience doing veterinary care for animals, does not have the experience to do brain surgery. Cassie performs major surgery in a later book as well, and neither book includes realistic detail about blood loss or any mention of anesthesia. Despite being an alien and in a delusional state, there's no reason Ax would not have had some realistic pain reaction to having his head drilled into.

Best lines:

Cassie: I'm not beautiful like Rachel. And I admit I sometimes have a little bird poop on my jeans.